Hydrogen Bonding Behavior of Amide-Functionalized α-Diimine Palladium Complexes

Playing with Pearson's concept: orthogonally functionalized 1,4-diaza-1,3-butadienes leading to heterobinuclear complexes

By reacting 1,2-diketones and ortho- diphenylphosphinoyl aniline in the presence of zinc(ii) as a templating agent, cationic zinc(ii) complexes of novel phosphine oxide functionalized 1,4-diaza-1,3-butadiene ligands are acessible. Herein the zinc(ii) site is bound to all four donor atoms of the ligand. Depending on the flexibility of the 1,4-diaza-1,3-butadiene backbone, the bonds to zinc(ii) from the 1,4-diaza-1,3-butadiene donors can be broken. Reaction with oxalate cleaves the zinc(ii) coordination completely and makes accessible the free ligands possessing orthogonal (N,N: soft; O,O: hard) sets of donor sites. This allows for the specific coordination of soft and hard Lewis acids and thus for the generation of heterobimetallic complexes, here exemplarily shown for the combination of palladium(ii) (soft) and zinc(ii) (hard) centres.

Critical Role of Anions in Platinum(II) Precursors upon the Structural Motifs of Six-Membered Cycloplatinated N,N',N″-Triarylguanidines

The reactions of cis-[Pt(OAc)₂(DMSO)₂] with 2 equiv of sym N,N',N″-triarylguanidines, [ArN=C(NHAr)₂], in toluene under reflux condition for 8 h afforded six-membered cycloplatinated guanidines, [Pt{κ²(C,N)}(OAc){κ¹ N(ArN=C(NHAr)₂)}] [sym = symmetrical; Ar = 2-MeC₆H₄ (1) and 2,4-Me₂C₆H₃ (2)], in 82 and 84% yields, respectively. The salt metathesis reaction of 1 with 1 equiv of AgTFA in CH₂Cl₂ at room temperature (RT) afforded [Pt{κ²(C,N)}(TFA){κ¹ N(ArN=C(NHAr)₂)}] (3) in 94% yield. The reaction of cis-[Pt(TFA)₂(DMSO)₂] with 1 equiv of [ArN=C(NHAr)₂] in toluene under reflux condition for 8 h afforded six-membered cycloplatinated guanidines, [Pt{κ²(C,N)}(TFA)(DMSO)] [Ar = 2-MeC₆H₄ (4), 4-MeC₆H₄ (5), 2,4-Me₂C₆H₃ (6), and 2-(MeO)C₆H₄ (7)], in ≥73% yields. The reaction of trans-[PtCl₂(PhCN)₂] with 2 equiv of [ArN=C(NHAr)₂] in toluene under reflux condition for 48 h afforded trans-[PtCl₂{ArN=C(NHAr)₂}₂] [Ar = 2-MeC₆H₄ (8) and 2,4-Me₂C₆H₃ (9)] in 90 and 45% yields, respectively. Complexes 8 and 9 were separately refluxed in MeOH for 8 h to afford six-membered cycloplatinated guanidines, [Pt{κ²(C,N)}(μ-Cl)]₂ (10 and 11), in 93 and 96% yields, respectively, with concomitant formation of the respective guanidinium salts, [(ArNH)₃C]Cl, as the byproduct. Platinacycle 10 was treated with 2 equiv of AgTFA in CH₂Cl₂ at RT to afford six-membered cycloplatinated guanidine, [Pt{κ²(C,N)}(μ-TFA)]₂ (12), in 94% yield. The new compounds were characterized by analytical techniques and multinuclear NMR (¹H, ¹³C, and ¹⁹⁵Pt) spectroscopy, and further, molecular structures of 10 compounds were determined by single-crystal X-ray diffraction. The structural motif in 1·1/2CH₂Cl₂ and 3 is novel in that it contains a planar six-membered [Pt{κ²(C,N)}] unit and a nonplanar eight-membered [Pt{κ²(N,O)}] ring, wherein OAc and the guanidine ligands are linked through a N-H···O hydrogen bond. The six-membered cycloplatinated structural motifs present in 10/11·C₇H₈ and 12·CH₂Cl₂ are also unprecedented in the literature. The number and nature of solution species of new complexes were unambiguously investigated by detailed NMR studies. The critical role of anions in Pt(II) precursors upon the course of cycloplatination and thus the motifs in the products were addressed. Plausible mechanisms of cycloplatination reactions are discussed.

Autoxidation of Heterocyclic Aminals

The autoxidation reactions of 2-acyl-2,3-dihydroquinazolin-4(1H)-ones 4a and 5a and 2,2'-bis(dihydroquinazolinone) 6a are described. These reactions generate aminyl radicals that undergo β-C-C cleavage, and subsequent reactions of the resulting C-based radicals with O₂ lead to diverse products with good selectivity, depending on the structure of the substrate. Oxidation of 4a, in which the 2-acyl group is part of a cyclic acenaphthenone unit, yields a heterocyclic C-hydroperoxylaminal via 1,2-acyl migration. Oxidation of 5a, which contains a 2-acetyl group, yields peracetic acid and a quinazolinone product. Oxidation of 6a forms a bis(quinazolinone) by net dehydrogenation.

Highly Robust Palladium(II) α-Diimine Catalysts for Slow-Chain-Walking Polymerization of Ethylene and Copolymerization with Methyl Acrylate

A series of sterically demanding α-diimine ligands bearing electron-donating and electron-withdrawing substituents were synthesized by an improved synthetic procedure in high yield. Subsequently, the corresponding Pd complexes were prepared and isolated by column chromatography. These Pd complexes demonstrated unique properties in ethylene polymerization, including high thermal stability and high activity, thus generating polyethylene with a high molecular weight and very low branching density. Similar properties were observed for ethylene/methyl acrylate copolymerization. Because of the high molecular weight and low branching density, the generated polyethylene and ethylene/methyl acrylate copolymer were semicrystalline solids. The (co)polymers had unique microstructures originating from the unique slow-chain-walking activity of these Pd complexes.